Change resolution of the video sequencer preview window
You can change the scale of the video preview display by selecting a different zoom level in the view menu. You can also use the mouse wheel to zoom in/out. This may not speed up the video display though as the same video still needs to be read and decoded.



To speed up video editing blender uses a proxy system, which you can find in the VSE properties region (press N). Creating a proxy will generate a set of reduced resolution images that will be used for the video preview. When it comes to the final render the full resolution source will be used.